Heat exchangers are well known in the prior art and typically include a pair of headers between which extend a plurality of tubes, and a plurality of fins interleaved between the tubes. An exemplary heat exchanger of the prior art is illustrated, partially conceptually, in FIG. 1. Often, the tubes are extruded elements which are fitted into apertures formed in the headers and sealed thereto by flux brazing. The tubes can be multi-port aluminum extrusions, having a plurality of discrete fluid passages defined lengthwise of the tube. The tubes can also be seam-welded tubes or extruded tubes having a single longitudinal interior passage. In the latter case, to improve heat transfer characteristics it is known to push or draw through the tube an elongate turbulizer of expanded metal or the like. The headers are often formed of a tubular part in which the apertures are defined and which is defined by a pair of trough-shaped or channel elements, and a pair of caps which close the ends of the tubular part. Often, the trough-shaped elements and caps are brazed or welded together. The trough-shaped elements are often of differing dimensions, such that the longitudinal peripheral edges of one of the trough-shaped elements fit within the longitudinal peripheral edges of the other, as shown in FIG. 2. This facilitates fixturing during the brazing operation. The channel elements can be self-fixturing, i.e. snap-fittable within one another, or an external fixture can be utilized.